fix(minify): log warnings to gulplog

If warnings are enabled, log them to the gulplog. The logging (and the
level of logging) will then be controlled by the gulp cli.

Fixes #163.

README.md

@@ -37,10 +37,8 @@ the UglifyJS API are supported. There are a few exceptions:
 
 1. The `sourceMap` option must not be set, as it will be automatically configured
    based on your Gulp configuration. See the documentation for [Gulp sourcemaps][gulp-sm].
-2. Setting the `warnings` option has no visible affect. [See #163](gh-163).
 
 [gulp-sm]: https://github.com/gulp-sourcemaps/gulp-sourcemaps#usage
-[gh-163]: https://github.com/terinjokes/gulp-uglify/issues/163
 
 ## Errors
 

minifier.js

@@ -57,6 +57,12 @@ module.exports = function(opts, uglify) {
       );
     }
 
+    if (mangled.warnings) {
+      mangled.warnings.forEach(function(warning) {
+        log.warn('gulp-uglify [%s]: %s', file.relative, warning);
+      });
+    }
+
     file.contents = new Buffer(mangled.code);
 
     if (file.sourceMap) {